nanomaterials Article Thermal Transport Evolution Due to Nanostructural Transformations in Ga-Doped Indium-Tin-Oxide Thin Films Alexandr Cocemasov 1 , Vladimir Brinzari 1, Do-Gyeom Jeong 2, Ghenadii Korotcenkov 1 , Sergiu Vatavu 3 , Jong S. Lee 2 and Denis L. Nika 1,* 1 E. Pokatilov Laboratory of Physics and Engineering of Nanomaterials, Department of Physics and Engineering, Moldova State University, MD-2009 Chisinau, Moldova; [email protected] (A.C.); [email protected] (V.B.); [email protected] (G.K.) 2 Laboratory for Spectroscopy of Condensed Matter Physics, Department of Physics and Photon Science, Gwangju Institute of Science and Technology, Gwangju 61005, Korea; [email protected] (D.-G.J.); [email protected] (J.S.L.) 3 Physics of Semiconductors and Devices Laboratory, Department of Physics and Engineering, Moldova State University, MD-2009 Chisinau, Moldova; [email protected] * Correspondence: [email protected]; Tel.: +373-675-60423 Abstract: We report on a comprehensive theoretical and experimental investigation of thermal con- ductivity in indium-tin-oxide (ITO) thin films with various Ga concentrations (0–30 at. %) deposited by spray pyrolysis technique. X-ray diffraction (XRD) and scanning electron microscopy have shown a structural transformation in the range 15–20 at. % Ga from the nanocrystalline to the amorphous phase. Room temperature femtosecond time domain thermoreflectance measurements showed nonlinear decrease of thermal conductivity in the range 2.0–0.5 Wm−1 K−1 depending on Ga doping Citation: Cocemasov, A.; Brinzari, V.; level. It was found from a comparison between density functional theory calculations and XRD Jeong, D.-G.; Korotcenkov, G.; Vatavu, data that Ga atoms substitute In atoms in the ITO nanocrystals retaining Ia-3 space group symmetry. S.; Lee, J.S.; Nika, D.L. Thermal The calculated phonon dispersion relations revealed that Ga doping leads to the appearance of Transport Evolution Due to hybridized metal atom vibrations with avoided-crossing behavior. These hybridized vibrations Nanostructural Transformations in possess shortened mean free paths and are the main reason behind the thermal conductivity drop Ga-Doped Indium-Tin-Oxide Thin in nanocrystalline phase. An evolution from propagative to diffusive phonon thermal transport in Films. Nanomaterials 2021, 11, 1126. ITO:Ga with 15–20 at. % of Ga was established. The suppressed thermal conductivity of ITO:Ga thin https://doi.org/10.3390/nano 11051126 films deposited by spray pyrolysis may be crucial for their thermoelectric applications. Academic Editor: Alex Trukhanov Keywords: thermal transport; indium-tin-oxide; thin film; thermoelectrics Received: 24 March 2021 Accepted: 23 April 2021 Published: 27 April 2021 1. Introduction Indium oxide-based compounds and composites with high thermal stability have been Publisher’s Note: MDPI stays neutral intensively investigated as prospective semiconducting materials for high-temperature with regard to jurisdictional claims in thermoelectric applications. Additives or supplement oxides of different metallic nature published maps and institutional affil- like Zn, Sn, Ge, Ga, Y, Nb, Ni etc. were used [1–7] in such compounds and composites iations. to improve their thermoelectric and optical properties. Some of these materials retained the lattice structure of the host oxide at large amounts of additives. The specifics of In2O3 lattice are its complexity and large number of atoms in the primitive cell (40). As a consequence one could expect the reduction of phonon thermal conductivity kph due Copyright: © 2021 by the authors. to the trapping of heat energy by numerous optical modes and flattening of their energy Licensee MDPI, Basel, Switzerland. dispersions [8], leading to lower phonon group velocities. The structural complexity of Ia-3 This article is an open access article space group impedes investigations of various defect complexes in In2O3 and their impact distributed under the terms and on electronic, phononic and thermoelectric properties. In Ref. [9], some of us reported on conditions of the Creative Commons the theoretical investigations of electronic properties of In2O3 with Sn-, Ga- and O-based Attribution (CC BY) license (https:// point defect complexes, employing a density functional theory. It has been shown that creativecommons.org/licenses/by/ defect complexes strongly influence the electronic band structure and position of indium 4.0/). Nanomaterials 2021, 11, 1126. https://doi.org/10.3390/nano11051126 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 1126 2 of 14 and oxygen atoms [9]. Defect complex driven deviation in both atomic mass and crystal lattice strain results in enhancement of phonon scattering by point defects and may affect the lattice thermal conductivity. The impact of different defect complexes and structural vacancies on thermal properties of In2O3 requires additional investigations. The strong improvement of specific electronic, thermal and thermoelectric properties could be reached when the material is in nanocrystalline form. Recent theoretical results demonstrated that enhanced electrical conductivity s and thermopower S providing an elevated power factor PF = sS2 can be achieved in ITO nanofilms [10] due to the filter- ing effect [10,11] of low energy conduction electrons at grain boundaries. PF values of around 3 mW·m−1·K−2 were reported for thermally aged ITO films with an optimal Sn content [12,13]. Thorough independent experimental study [14] confirmed the presence of potential barrier in the vicinity of grain boundaries which are responsible for the filtering of electrons despite the degeneracy of ITO conduction band. ITO nanofilms have also demonstrated reduced values of thermal conductivity as compared with bulk ITO [15]. The drop of thermal conductivity was explained by strong phonon scattering on grains, reduction of electronic part kel of thermal conductivity due to the filtering of low energy conduction electrons and porosity of ITO films [15]. Due to the geometric complexity of the nanograin network, the trapping of heat carrying vibrational modes leading to the decrease of phonon thermal conductivity kph is also possible. Reduction of thermal conductivity without degradation of electronic parameters (s, S) radically improves the thermoelectric 2 efficiency, determined by figure of merit ZT = sS T/ktot. Looking for compounds with low ktot and high enough s and S is a mainstream in thermoelectric material science. The strong suppression of heat conduction in different nanolayers, nanowires and superlattices due to phonon interface scattering and/or spatial confinement of phonons was reported in numerous theoretical and experimental studies [16–20]. Our experiments with ITO:Ga thin films deposited by spray pyrolysis have shown quite large values of PF [13]. However, reported values of electrical conductivity were smaller than those in undoped ITO due to the peculiarities of spray pyrolysis technique used, leading to the formation of finely dispersed Ga2O3 grains on the main crystallite surfaces [13]. At the same time the influence of deposition parameters (like as pyrolysis temperature) on electric and thermal properties of such films requires further investigations and optimization. ITO films remain totally nanocrystalline up to 50 at. % Sn [12], while nanocrystalline to amorphous phase transition in ITO(Sn ≤ 10 at. %):Ga-based films occurs at Ga content ≥ 20–30 at. % and may lead to a sharp drop in the thermal conductivity. Thereby, in this work we focus on experimental and theoretical investigations of thermal conductivity in ITO:Ga thin films with widely varying Ga content and almost constant amount of Sn ~6 at. %. Such concentration of Sn [12,13] is optimal in terms of PF and ZT. The experiments were performed by an ultrafast (femtosecond) laser-induced time domain thermoreflectance method (TDTR) [21,22]. It is a powerful and versatile technique for thermal properties investigations of a large variety of bulk and nanoscale systems. Within the many advantages of TDTR in comparison with conventional thermal conductivity measurements is an excellent spatial resolution at a length scale below tens of micrometers. TDTR also requires minimal sample preparation for the measurements. Using density functional theory (DFT) and linearized Boltzmann transport equation (BTE) for phonons, we investigate in detail how changes in phonon energy spectra affect both phonon scattering mechanisms and lattice thermal conductivity in ITO:Ga films at different Ga concentrations. The rest of the paper is organized as follows. In Section2 we discuss sample prepa- ration and characterization. Section3 describes TDTR technique of thermal conductivity measurements. Our theoretical model used for the calculations of phonon modes and lattice thermal conductivity in ITO:Ga films is presented in Section4. Discussions of results are provided in Section5. Conclusions are given in Section6. Nanomaterials 2021, 11, 1126 3 of 14 2. Sample Preparation and Characterization ITO films (6 at. % of Sn content in sprayed solution) were deposited at T = 350–360 ◦C by spray pyrolysis method on polished silicon substrates (1 × 1 cm2). Additionally, within the same deposition procedure, several films were doped by Ga at various concentrations in the range up to 30 at. %. Hereinafter, under Ga concentration we refer to the following ratio: [Ga]/([In] + [Sn] + [Ga]) where corresponding partial concentrations of compound are indicated in parentheses. The sprayed solution was prepared as a mixture of